There is a conventional spectroscopic analyzer which includes: as shown in Patent Literature 1, a light source; a condensing lens condensing light from this light source; a spectroscopic analysis unit having a multi-channel detector (light detector) and which performs spectroscopic analysis of the light from the light source; and a measurement cell being arranged between the condensing lens and the spectroscopic analysis unit (see FIG. 9).
An absorption spectroscopy method is used for concentration measurement in this spectroscopic analyzer. Typically, in this absorption spectroscopy method, concentration cj is typically calculated by multiplying an absorbance spectrum Abs (λi) by a calibration curve Mij previously obtained through calibration (see equation below). λ1, λ2, . . . λn are measurement wavelengths and cj is concentration of the j-th component.
                              [                      Equation            ⁢                                                  ⁢            1                    ]                ⁢                                                                                      cj        =                              ∑                          i              =              1                        n                    ⁢                      Mij            ×                          Abs              ⁡                              (                                  λ                  ⁢                                                                          ⁢                  i                                )                                                                        (                  Equation          ⁢                                          ⁢          1                )            
The absorbance spectrum Abs (λi) can be expressed by the following equation based on intensity I0 (λi) of light entering the measurement cell and intensity Is (λi) of light transmitted through the measurement cell.
                              [                      Equation            ⁢                                                  ⁢            2                    ]                ⁢                                                                                                Abs          ⁡                      (                          λ              ⁢                                                          ⁢              i                        )                          =                  log          ⁢                                          ⁢                                                    I                0                            ⁡                              (                                  λ                  ⁢                                                                          ⁢                  i                                )                                                                    I                S                            ⁡                              (                                  λ                  ⁢                                                                          ⁢                  i                                )                                                                        (                  Equation          ⁢                                          ⁢          2                )            
Here, it is difficult to directly measure the intensity I0 (λi) of the entering light, and with the conventional spectroscopic analyzer, in a state in which a measurement lens is removed from between the condensing lens and the spectroscopic analysis unit (reference measurement), intensity of light measured by the spectroscopic analysis unit, that is, intensity IR (λi) of reference light is substituted. That is, with the following equation, the absorbance spectrum Abs (λi) is obtained. In the spectroscopic analyzer of Patent Literature 1, optical glass of, for example, quartz is arranged for the purpose of correcting a change in a focal position caused by the presence or absence of the measurement cell on a light path (see FIG. 9).
                              [                      Equation            ⁢                                                  ⁢            3                    ]                ⁢                                                                                                Abs          ⁡                      (                          λ              ⁢                                                          ⁢              i                        )                          =                  log          ⁢                                                    I                R                            ⁡                              (                                  λ                  ⁢                                                                          ⁢                  i                                )                                                                    I                S                            ⁡                              (                                  λ                  ⁢                                                                          ⁢                  i                                )                                                                        (                  Equation          ⁢                                          ⁢          3                )            
Moreover, with the spectroscopic analyzer of this kind, a measurement wavelength and a light path length of the measurement cell are typically set so that absorbance when a liquid sample is stored in the measurement cell becomes approximately 1 Abs to 2 Abs (transmittance of approximately 10% to 1%) where absorbance reference (an absorbance of zero) is provided for the case where air is stored in the measurement cell. This is because for an absorbance of 1 or below (transmittance of 10% or above), absorbance variation corresponding to concentration variation of the liquid sample is small and thus concentration measurement cannot be performed with high accuracy. On the other hand, for an absorbance of 2 or above (transmittance of 1% or below), an intensity of transmitted light is small and thus it is difficult to perform accurate light intensity measurement and also perform concentration measurement with high accuracy.
However, transmittance of optical glass used for the reference measurement is approximately 90%, and thus a ratio of intensity of the reference light (an output value of an analog-to-digital [A/D] converter) with respect to intensity of the sample light (an output value of the A/D converter) becomes approximately ten times to hundreds of times as shown in FIG. 2. The sample light and the reference light are detected by the same optical detector, and their light intensity signals are amplified by the same amplifier and are converted by the same A/D converter, and thus as a result of a large difference in light intensity signals between the sample light intensity and the reference light intensity, disadvantages arise in both the signal-to-noise (S/N) ratio and the light intensity resolution.
Specifically, since the intensity of the sample light is small, the light intensity signal outputted from the optical detector is small, deteriorating the S/N ratio due to an influence from disturbance noise such as electromagnetic noise. Moreover, a full scale of the A/D converter is set in accordance with the reference light with large intensity, and thus the sample light with small intensity needs to be subjected to A/D conversion within a narrow range of the A/D converter. This means that the intensity of the sample light needs to be measured with a rough bit, and light intensity resolution provided by the A/D converter decreases.